A coated type magnetic recording medium that has been widely used is prepared by dispersing magnetic oxide particles such as .gamma.--Fe.sub.2 O.sub.3, Co-doped .gamma.---Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-doped Fe.sub.3 O.sub.4, a Berthollide compound of .gamma.--Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, a Co-doped Berthollide compound or CrO.sub.2, or magnetic alloy particles mainly composed of Fe, Co, Ni and the like in an organic binder such as a copolymer of vinyl chloride and vinyl acetate, a copolymer of styrene and butadiene, an epoxy resin or a polyurethane resin, coating the resulting coating composition on a non-magnetic support, and drying it.
With the increased demand for high density recording, a thin metal film type magnetic recording medium, of which ferromagnetic metal thin film is prepared by vapor deposition, sputtering, or ion plating without using a binder has drawn attention and has been developed to put into practice.
A conventional coated type magnetic recording medium uses magnetic metal oxide particles having low saturation magnetization. Therefore, when a magnetic recording layer is made thinner in order to realize high density recording, the output of the signals is decreased.
On the other hand, a thin metal film type magnetic recording medium uses ferromagnetic particles having higher saturation magnetization than magnetic metal oxide particles in the absence of a binder to prepare a magnetic recording layer. Therefore, the resulting magnetic recording layer can be made extremely thin and exhibits excellent electromagnetic properties.
Upon preparing a magnetic tape by forming a magnetic film on a tape-shape non-magnetic support by a vapor deposition, a polyester film is widely used as a non-magnetic support.
When a magnetic tape is prepared on a polyester film by a vapor deposition method, there are problems as to characteristics of a magnetic tape due to heat treatment at vapor deposition. One of the problems is a skew characteristic (i.e., a distortion of an image which is caused by shrinkage of tape in the running direction). It is proposed in Japanese Patent Publication No. 202794/85 to conduct heat treatment after vapor deposition in order to overcome the above problem, however, there is also a problem in this method that a crack is formed on a vapor deposited film. It is also proposed in Japanese Patent Application (OPI) No. 135632/84 (the term "OPI" as used herein means an "unexamined published application") that a coated type magnetic layer is provided on a film after the polyester film is subjected to heat treatment. However, in this method, skew problems cannot be overcome, and electromagnetic properties are also deteriorated, because in a magnetic recording medium prepared by a vapor deposition method, the film undergoes heat treatment upon vapor deposition.